Shirt chest measurement compared to actual chest measurement

I will be having some shirts made as well as having some tailored. I wanted to know what you guys typically have for a difference in the measurement of your shirt chest to the actual chest size. For example, I have a 41" chest and am contemplating having my shirts taken to a chest size of 21.5" (43 inches total). This would be a difference of 2". I am looking for a trim fit but still being able to breath.

On propercloth they recommend to "divide your actual chest measurement by two and add 3 inches" for shirt chest width. For me, this results in 37"/2 + 3" = 21.5". Now, I can tell you right now that I SWIM in a shirt that measures 21.5" across the chest. Where are they getting this?? Even adding 4-5" as the other posters suggested gives (37" + 4") / 2 = 20.5". Do any of you skinny guys have more input for myself and the OP on what the standard amount to add for a slim (but properly) fitting shirt should be? I really feel like my shirts look best with a width of around 20", but perhaps I am buying them too tight...